A prior art air-puff tonometer of such type is shown in FIG. 7.
The tonometer shown in FIG. 7 has a rotary solenoid 1 which acts as means for driving a piston 3. An arm 2 of the rotary solenoid 1 is connected to a piston rod 4 integral with the piston 3. The piston 3 is mounted in a cylinder 5 in which a compression chamber 6 is defined. The chamber 6 is connected to a nozzle (not shown) which is used to direct a puff of compressed air supplied from the compression chamber 6. The arm 2 will be turned in the counterclockwise direction by energizing the rotary solenoid, and will be turned back to the initial position by the elastic action of a spring (not shown) upon deenergization of the solenoid 1.
As the arm 2 is turned in the counterclockwise direction by the rotary solenoid 1, the piston 3 is moved upward with the piston rod 4, whereby air within the compression chamber 6 is compressed and thus driven against the subject eye through said nozzle.
To blow out a puff of compressed air, a strong force must be applied to the piston 3 and therefore an intense electric current must be supplied to the rotary solenoid 1 through a constant-current circuit 9. The intense current is generated by discharging a capacitor 8 in a short time. The capacitor is charged by a charging circuit 7.
The constant-current circuit 9 regulates the intense current to a constant current, which is supplied to the rotary solenoid 1, whereby the solenoid 1 will drive the piston 3 with a force of predetermined strength. The pressure of the air puff is thus adjusted to a constant value.
A prior art air-puff tonometer disclosed in Japanese patent publication No. 38437/1979 is provided with an optical transmitter system (not shown) for directing a detection light beam to the cornea of a subject eye, and with an optical receiver system (not shown) for receiving the detection light which was emitted from the transmitter and reflected by the cornea. This prior art apparatus directs a flow of fluid to the cornea of the subject eye. The cornea will be deformed from convex to flat and then to concave as the pressure of the fluid flow increases. Further, the cornea will then be deformed from concave to flat, and will eventually restore its original convex form as the fluid pressure decreases. The air-puff tonometer operates based on such deformation of the cornea.
The optical transmitter and the optical receiver are set such that the amount of light received by the receiver will be maximum when the cornea of the subject eye is flat. The compressing of air is performed such as to achieve a predetermined pressure-time characteristic curve A shown in FIG. 8. Since the time required for the deformation or restoration of the cornea is correlated to the intraocular pressure, it can be determined using such time as the variable. In accordance with the diclosure of the above-mentioned Japanese patent publication No. 38437-1979, the time t.sub.1 is measured which is the interval of time from the initiation of the fluid flow according to the pressure-time characteristic curve A until the cornea is deformed from convex to flat, and the thus measured time t.sub.1 is converted to the corresponding intraocular pressure. In fact, the time t.sub.1 is measured as the time interval until the intensity of light reflected from the cornea and received by the receiver system becomes maximum. The conversion of the time t.sub.1 is as follows: the pressure value of the fluid Pe at the point of time t.sub.1 is determined on the basis of the fluid pressure-time characteristic curve A and such value Pe is considered to be equal to the intraocular pressure. In accordance with such prior art, the intraocular pressure can also be determined using a measured time interval between a first time point t.sub.1 at which the cornea becomes flat while being deformed inwardly by the fluid flow directed thereto, and a second time point t.sub.2 at which the cornea becomes flat again while being restoring its original convex form. In FIG. 8, character B designates a characteristic curve of the amount of received light versus time, while P.sub.max designates the maximum pressure value of the fluid.
The first mentioned prior art air-puff tonometer however has a problem in that a change in the clearance between the piston 3 and the cylinder walls due to abrasion will cause a corresponding change in the pressure of air compressed in the cylinder 5 since the air is compressed by the piston 3 being driven with a predetermined constant thrust produced by the constant current supplied from the constant-current circuit 9 to the rotary solenoid 1. Since said change in the clearance will cause a corresponding change in the pressure-time characteristic of the air puff blown out through the nozzle, significantly large errors may be included in the measurements taken by a tonometer which determines the intraocular pressure on the basis of the measured time interval between the moment the air puff occurs and the moment the cornea flattens.
The second mentioned prior art air-puff tonometer disclosed in Japanese patent publication No. 38437/1979 also has a problem in that it does not actually measure the air puff pressure at the moment the cornea flattens. Thus, it is necessary for such tonometer that the air-puff pressure should vary following the pressure-to-time characteristic curve A of FIG. 8. Therefore, any deviation of the actual air-puff pressure from the reference pressure-time characteristic curve A will immediately cause a corresponding error in the value of the intraocular pressure thus determined by the tonometer. The prior art tonometers thus are not ones which allow for improvent in the accuracy of measurent.